Semiconductor devices are used in integrated circuits for a wide variety of electronics applications, such as televisions, radios and cellular phones, as examples. There is a trend in the electronics industry towards miniaturization of electronic components, so that electronic devices can be made smaller and lighter. Integrated circuits may comprise multiple levels of metallization for interconnections, and multiple layers of dielectric and semiconductor materials. These layers are patterned using lithography to form conductive lines, transistors, and other elements of integrated circuits.
For a long time, silicon dioxide has been commonly used as a dielectric to isolate conductive lines and various components within the various layers. More recently, low-K materials (e.g., materials having a low dielectric constant K) are being used as insulators in semiconductor devices. Using low-k materials provides the benefit of reduced RC time constants of the metallization layers and lower cross-talk, leading to faster speeds and smaller devices.
However, low-K dielectric materials typically have a low modulus and are structurally weak compared to silicon dioxide. The modulus of a material refers to the Young's modulus or elastic modulus, which is a constant for any given material up to its elastic stress limit. The elastic modulus of a material indicates the linear deflection of a material under normal stress, and the modulus is represented by a ratio of this normal stress to linear strain. Low modulus dielectric materials strain and deform easily, thus providing less structural stability for metallization embedded in the low-k dielectric.
An integrated circuit chip or die includes contact pads that are wirebonded, typically using gold wire, to contacts of a package to allow electrical connection to the die when the IC is packaged. Wirebonding typically comprises a welding process, wherein heat is applied to the contact pads and wires to ensure a good connection. Some welding processes include an ultrasonic process that vibrates the wire laterally being bonded to the contact pad, and also applies a downward pressure against the contact pad.
Wirebonding contact pads having low-K dielectric materials disposed beneath the pads is problematic. In the wirebond process, some of the welding stress is transmitted down into the structure of the die below the contact pad. This mechanical stress from the wirebond welding process can damage the contact pads and underlying structures, and cause the contact pads to lift or tear from the silicon substrate, destroying the semiconductor device and decreasing yields. Because low-K materials have a low modulus and are structurally weak, the die is more susceptible to welding damage, resulting in high levels of mechanical failure due to lifts and tears of the contact pads.